Reactive ion etching is usually performed in a chamber designed to hold a single silicon wafer. With the wafer in place, a reactive gas is admitted into the chamber and a high density plasma discharge is initiated, most commonly in the form of a TCP (transformer coupled plasma). Typically the power level that is used is about 200 watts and etching is allowed to proceed for about 5 minutes. This apparatus may be used over and over again for different wafers but, over a period of time, a layer of material that is a byproduct of the etch reaction builds up on the walls of the chamber. Eventually this layer grows to be thick enough to be a problem because of particulate matter that flakes off its surface.
Thus, it becomes necessary to periodically clean the inside walls of the chamber. This is typically done using some sort of dry cleaning process. Once the walls have been cleaned the chamber may be used again for the reactive ion etching of many more wafers. However it turns out that the electrical characteristics of the chamber are altered by this cleaning process and do not return to normal until the unit has been run in etch mode for a certain amount of time. To avoid wasting product wafers it is standard practice to' season' a freshly cleaned chamber using blank wafers.
Prior to the present invention it was found necessary to use as many as 12 wafers before a chamber could be considered to be satisfactorily seasoned. Seasoning was performed in three steps as listed in TABLE I below:
TABLE I flow rate RF power in watts of gases (SCCM) pressure time in source bias Cl.sub.2 HBr He--O.sub.2 mTorr seconds 250 170 50 -- 2 4 40 250 200 50 150 -- 10 40 350 180 -- 270 3.3 60 60
Thus, since the wafers had to be seasoned in sequence (since a chamber holds only one wafer at a time) this implies a total time of at least 28 minutes to complete the seasoning process. In the context of a production line this represents a significant downtime for a key component of the line.
In FIG. 1 we illustrate, schematically, the apparatus used to effect the seasoning process. Wafer 1 is seen present inside etch chamber 2 in which a plasma (symbolized by arrows 3) has been initiated through RF excitation by Coil 8. The progress of the seasoning process is most easily monitored by observing the optical emission spectrum of the plasma As we will discuss below, light from the chamber is directed along glass fiber 4 where it is detected by photo-multiplier tube 5 after it has been analyzed by a suitable device such as a monochromator.
Since chlorine is present in the reactive gas it is convenient to monitor the intensity of the silicon chloride species SiCl.sub.x in the gas. Spectral lines associated with this radical occur at 280.4 and 282 nm. In FIG. 2 a series of spectra obtained by monitoring the plasma during seasoning are shown. Curve 21 shows the spectrum seen immediately after dry clean. Curve 22 is the spectrum seen during seasoning by the second wafer and so on for a succession of wafers until there is no further rise in the intensity beyond curve 23 which would be obtained during seasoning with the 12th wafer. A similar asymptotic increase in intensity for successive wafers is seen at wave lengths 288 nm which is due to silicon.
The present invention teaches a method that uses only one wafer for achieving the same results previously obtained with 12 wafers. In the course of searching the prior art no relevant references to the process described in the present invention were found. One or two references of interest were, however, encountered. Thus, Wu et al. (U.S. Pat. No. 5,585,012) keep the top electrode of their etcher under weak RF power, either continuously or in spurts, thereby removing deposited material through RF sputtering as soon as it forms and thus eliminating the need to dry clean the unit.
Ye et al. (U.S. Pat. No. 5,756,400) give a description of the dry cleaning procedure itself Said procedure is very short and is applied at frequent intervals so that relatively little downtime results. Keller et al. (U.S. Pat. No. 5,256,245) also described a dry cleaning procedure based on using an oxygen scavenging gas together with a volatile fluoride. Murugesh et al. describe a seasoning process wherein higher than customary bias RF power is used with the time for seasoning being increased slightly. Their recommended cleaning gases include NF.sub.3, CF.sub.4, and C.sub.2 F.sub.6. Seasoning in their case was used to prepare the chamber for a CVD process rather than for sputter etching.